Using Memory Diagrams When Teaching a Java-Based CS1 Mark A. Holliday David R. Luginbuhl Dept of Mathematics and Computer Science Western Carolina University ACM-SE Conference March 7, 2003 Savannah, GA

3/7/2003ACM-SE Overview  Motivation and Introduction  Diagramming as a teaching tool – Objects, variables, and references – Primitive types, fields, visibility, and parameter passing – Static fields and methods – Arrays  Diagramming as an assessment tool  Conclusion

3/7/2003ACM-SE Motivation  Problem: – How to help CS1 students visualize state of computer as Java program executes – How to introduce and reinforce object-oriented concepts in an introductory CS course  Solution: – Diagrams Abstraction; visualization – “UML”-like UML not primarily focused on state of memory – Diagrams used in many textbooks, but without much emphasis

3/7/2003ACM-SE Criteria  Take into account features of language (for us, Java)  Visualize state of computer  Distinguish between – Objects: classes: reference variables – Reference types: primitive types – Private: public  Consistency for similar concepts  Method invocation and object construction  Labels, as appropriate  Arrays vs. other objects

3/7/2003ACM-SE Memory Diagrams  Similar to other diagrams – See, particularly, [WU2001] – But with extensions and particular emphases  Important aspects – Shape matters – Importance of reference and invocation – Indication of values with variables – Visibility rules

3/7/2003ACM-SE Objects, Variables, and References String acolor=“blue”; String favColor=aColor; String otherColor; String “blue” aColor favColor otherColor Features:  Labels on objs and vars  Different shapes  Reference lines begin inside variable

3/7/2003ACM-SE Method Invocation aColor = aColor.concat(otherColor); String “blue” aColor otherColor String “red” concat ( ) String “bluered” X Features:  Method indicated by line on object – Indicates each object has method  Method is public  Squiggly line indicates invocation  New object created  New value for aColor; old value removed

3/7/2003ACM-SE Primitive Types, Fields, Multiple Instances  Primitive type variables have numeric value – As opposed to ref types  Private fields represented inside object – Using rectangles indicates fields are just variables associated with objects  Different Student objects have same fields with (possibly) different values Student first second Student 87 score 85 score name String “Sue” String “Bob”

3/7/2003ACM-SE Static fields and methods  Static fields and methods are displayed in a diamond instead of a circle – Punch clock provides a nice example for static fields and methods  See paper for more examples, including passing parameters and diagramming arrays Employee 8 16 clock Employee john 8 start 16 end advance (8)

3/7/2003ACM-SE Student Exposure to Diagrams  We introduce these diagrams to students on the first day of CS1 class  We ask students to produce diagrams of their own – In groupwork – In directed lab work – On quizzes and tests  Bottom line: reinforcement of concepts in a number of contexts – But not just for learning…

3/7/2003ACM-SE Student Assessment  By having students use diagrams themselves, we have them demonstrate their comprehension of object- oriented concepts  We believe these diagrams have potential for measuring programming comprehension – Anecdotal evidence from several semesters of application

3/7/2003ACM-SE Example Test Problem From Last Fall  Diagram the program fragment below Dog spot;// fig a) spot = new Dog("spot"); // right hand side is fig b) // left hand side and equal sign is fig c) System.out.println(spot.toString());// fig d)  Note last line particularly – Static public variable (System.out) – Method composition – PrintStream object – Dog object – Creation of a String object

3/7/2003ACM-SE Results (13 students)  Figure a – 11 correct – 2 labeled rectangle with name of class  Figure b – 4 correct – 2 put “spot” inside var rectangle – 4 didn’t show name field – 2 had more serious errors  Figure c – all but 1 student correct Dog spot;// fig a) spot = new Dog("spot"); // right hand side is fig b) // left hand side and equal sign is fig c) System.out.println(spot.toString()); // fig d)

3/7/2003ACM-SE Results (13 students)  Figure d – All but 3 students showed call to toString() correctly – Of those 10 2 students failed only to show System.out correctly 1 showed nothing else correctly 5 missed the creation of a String object from toString() – They got the println() call right 2 showed the String object – But they missed the println() call  Summary for this exam – “Decent” correlation between overall grades and grades on diagramming problem Dog spot;// fig a) spot = new Dog("spot"); // right hand side is fig b) // left hand side and equal sign is fig c) System.out.println(spot.toString()); // fig d)

3/7/2003ACM-SE Summary for Assessment  We have more work to do  We are considering a more rigorous examination of diagramming as a student assessment tool – Not just to measure progress – We hope we can pinpoint problems and provide proper reinforcement of specific concepts

3/7/2003ACM-SE Conclusion – Memory Diagrams  A relatively low-tech approach for teaching OO concepts – Well-suited for classroom, labs, exams – Importance of shape and placement for reinforcing concepts – Having students make their own diagrams adds to this reinforcement  Promise of diagrams for measuring comprehension – If students can diagram what is happening in memory, they are probably understanding the deeper meaning of the program – More work to do